Pulse selecting circuit for multiplex systems



Aug. 2e, v1947. un. GRIEG ETAL 2,426,205"

PULSE SELECTING CIRCUIT FOR MUL'I-IPLEX SYSTEMS Filed March 2, 1946 2sheets-sheet 1 (il P900/*Pur zal/WEG, cwi:

INVENToRS 2o/mm a., GR/EG HFA/OLD M. fV//VE ATTORNEY Patented Aug. 26,1947 PULSE SELECTING CIRCUIT FOR MULTIPLEX SYSTEMS Donald D. Grieg andArnold M. Levine, Forest Hills, N. Y., assignors to FederalTelecommunication Laboratories, Inc., corporation of Delaware New York,N. Y., a

Application March 2, 1946, Serial No. 651,652 6 Claims.k (Cl. 179-15)This invention relates to a circuit for selecting channels in amulti-channel pulse communication system in which the pulses are timemodulated, and for demodulating the pulses of the selected channel.

In communication systems of the type to which this application hasreference, the pulses forming the separate channels are interleaved andhave dilferent time displacements with respect to regularly repeatedmarker or synchronizing pulses. The marker pulses may be single pulseshaving a distinct characteristic, such as a unique width distinguishingthem from the signal pulses, or the marker pulses may consist of pairsof closely spaced pulses having a different time displacement from eachother than have the signal pulses. The signal pulses are time modulatedbut the eX-V tent of modulation is so related to the spacing of thepulses as to prevent a pulse of one channel from moving into a timeposition received for a pulse of another channel.

An object of the present invention is the provision of an improvedcircuit for selecting channels, and for demodulating the time modulatedpulses in the selected channels of a multi-channel pulse communicationsystem of the type hereinabove described.

Another object is the provision of a circuit, of the type hereinabovedescribed, which is characterized by its simplicity.

In accordance with a feature of the present invention, the circuithereinabove described acts as a demodulator by producing rectangularwaveforms of variable width (duration) corresponding to the timeelapsing between the marker pulses and the associated signal pulses ofthe channel to be selected. Each of these rectangular waveforms isinitiated terminated by the associated signal pulse of the selectedchannel. Consequently the width of each of these waveforms is a functionof the time modulation of the pulse terminating them. These variablewidth rectangular waveforms are applied to a suitable utilizationdevice. 'Ihe circuit also acts as a channel selector in that each of therectangular waveforms can only be terminated by a signal pulse occurringduring a given interval beginning at a selected time after the markerpulse, the duration of said interval being sulcient to include theextremes of time displacement of the pulse of the selected channel dueto the time modulation of said pulse.

Other and further objects of the present invention will become apparentand the invention will be best understood from the following deby amarker pulse and scription of an embodiment thereof, reference being hadto the drawings, in which:

Fig. 1 is a block diagram of a multi-channel communication receiveremploying selector circuits in accordance with the present invention;

Fig. 2 is a schematic diagram of one of the selector circuits, shown inblock diagram in Fig. I; and

Fig. 3 is a set of curves used in describing the operation of theselector circuit embodying our invention, these curves having no exactquantitative significance, but being introduced merely to enable easierunderstanding of the present invention.

In multi-channel communication systems of the type to which thisapplication has reference, a plurality of signal pulses, as for example,signal pulses I--8, each forming part of a separate channel, areinterleaved in sequence and progressively displaced in time with respectto an associated marker pulse 9. The marker pulse 9 is distinguishedfrom the signal pulses for eXample by having a different width (asillustrated in curve A, Fig, 3) or by some other distinguishing means.The marker or synchronizing pulse S may consist of two pulses having adifferent spacing therebetween than the spacing between the signalpulses. The marker pulses 9 are regularly repeated and so are the signalpulses I--8 except for the variation due to the time modulation of saidpulses within relatively narrow limits a and b. These limits a and b areso chosen that no pulse will overlap into a time Zone in which anotherpulse moves when it is modulated. Pulses l-9 may be generated, and thesignal pulses I- may be time modulated, in a manner known in the art.They may be then used to modulate a carrier and radiated, or carriedover a wire, or the pulses I-S may be transmitted to a receiver by anysuitable transmission medium. Pulses I-B are preferably all of the sameamplitudc.

Referring now to Fig. l, the incoming pulses may go through a receiverunit I 0 which may be used to assure that all the output pulses thereofare of equal amplitude. These equal amplitude pulses, similar to thoseillustrated in curve A (Fig. 3) are then fed to a plurality of channelsIi, I2, I3, etc., over lines I4, I5, I6, etc. The marker orsynchronizing pulses 9 are separated by a suitable marker separatorcircuit I 'I and fed over lines I8, I9 and 2l] into various channels II,I2, I3 respectively. The marker separator circuit II may be any suitableknown device Which utilizes the distinct characteristics of the markerpulses 3 to separate them from the signal pulses. For example, where, asillustrated, the marker pulses are distinguished in width from thesignal pulses, the separator I1 may be any known form of pulse Widthselector.

Since all the channels are similar, only channel II will be described.In channel II, there is provided a selector circuit 2| on which areimpressed the separated marker pulses over line I8, as well as all thepulses over line I4. The selector circuit produces a rectangularwaveform in response to pulses of the selected channel, the width ofsaid waveform varying in accordance with the time modulation of thepulse of the selected channel. These waveforms are then fed to asuitable integrating circuit 22 which may be for example, an audio lterand are thereafter fed to a suitable utilization device 23, which mayconsist for example, of audio amplifiers and a sound reproducer.

Referring now to Fig. 2, the selector circuit 2| is a multivibratorwhich is adapted to be tripped by a marker pulse and to be returned bythe following signal pulse of the selected channel. In the absence of asignal pulse in the selected channel (which in the systems to which thisapplication has particular reference only occurs inadvertently), themultivibrator will return to its ,original state or output voltage levelafter a time has elapsed from the initiation of its oscillation equal tothe maximum time between any synchronizing pulse and the most extremelydisplaced pulse of the selected channel. The constants of the circuitare moreover so chosen that the multivibrator cannot be returned by anypulse occurring before the minimum time elapsing between anysynchronizing pulse and any associated one of the pulses of the selectedchannel. In accordance with a further feature of the invention, thesynchronizing or marker pulses 9 each generate a blocking voltagewaveform which has a duration substantially equal to said minimum time,and which blocking voltage preventsI return of the multivibrator to itsoriginal level.

For the purposes of operation mentioned in the foregoing paragraph, themultivibrator 2| may include two electron tubes 24 and 25 each of whichmay be a triode and may be contained within a single envelope. Tube 24is normally blocked to cut-off as for example by means of a potentialderived from the positive side of a source 2S which is applied through asuitable resistor 2'! to the cathode of tube 24 and which may furtherinclude a cathode resistor 28 shunted by the usual by-pass condenser 29.The marker pulses 9 are fed to the grid of tube 24 over line I8 and asuitable grid resistor 30 is provided between line I8 and ground. Themarker pulses 9 applied to the grid of tube 24 are sufcient in amplitudeto cause said tube to conduct. Since these marker pulses are positive,the anode of tube 24 becomes negative. The anode of tube 24 is coupledover a variable condenser 3| to the grid of tube 25. A negative voltageis thereby impressed upon the grid of tube 25 which causes the anode ofsaid tube to become positive. The anode of tube 25 is connected over acoupling condenser 32 to the output. This the potential in the output ofmultivibrator circuit 2| rises steeply in a positive direction asillustrated at time T1 curve E in Fig. 3, time T1 corresponding to thetime when the marker pulse is impressed over line I8 upon the grid oftube 24. The anode of tube 25 is coupled by means of the 1usualcondenser 33 to the grid of tube 24. In accordance the known operationof multivibrators of the type hereindescribed, when tube 24 conducts,condenser 3I is charged negatively, preventing tube 25 from conducting.After a selected time has elapsed, depending upon the adjustment ofcondenser 3I, and its R.-C. discharge circuit which includes a variableresistor 34 and resistor 35 coupled to ground 36 in series with eachother, the charge leaks off the grid of tube 25, and tube 25 againconducts whereupon the anode of tube 25 becomes negative and the outputvoltage drops from the level it reached at time T1 back to its first ororiginal level.

In accordance with the present invention the time for a completeoscillation of multivibrator 2| is selected so that in the absence ofany signal pulse, its duration is equal to the maximum time displacementbetween a signal pulse of the selected channel and the marker pulse withwhich it is associated. For example, referring to curve A (Fig. 3),assuming that pulse 3 is the pulse to be selected, and that pulse 3 hasa maximum time displacement due to modulation between the limits 3a and3b, the oscillation of multivibrator 2| will terminate substantially ata time T4, corresponding to the maximum displacement 3b of pulse 3 frommarker pulse 9. Thus pulses occurring after time T4, will have no effectupon the rectangular wave form produced by the multivibrator since theirapplication at point I4 to the grid of tube 25, will have negligibleeffect especially in view of the fact that the voltage swing of themultivibrator may be, for example, of the order of volts, whereas theamplitude of the pulses may be for example, l0 or 20 volts. Moreoversince the pulses applied over line I4 cannot trigger the multivibrator,it will not be tripped again until the next marker pulse 9 arrives.

In accordance with a feature of the present invention, use is made ofone of the known characteristics of multivibrators of this type whichenables a pulse applied on line I4 to cause said multivibrator to returnto its original level before time T4. This characteristic is as follows:When a positive voltage is applied to the grid of tube 24, the anodethereof becomes negative and remains negative from time T1 when themarker pulse 9 is applied to the grid, until time T4 which is theselected period of vibration of the multivibrator as determined by theR.-C. constants of the circuit. Thus a voltage having a rectangularwaveform 3'! appears on the anode of tube 24 (see curve B, Fig. 3). Thegrid of tube 25 is coupled to the anode through condenser 3 I, but due,however, to the constants of the condenser 3| and its associatedresistors, a voltage having a waveform 38 appears at the grid of tube25, as indicated in curve B in dotted lines, said waveform having a flatportion 39 which starts to curve positively a short time before time T2to form a curve 40. The shape of the curve 4I) which is made as linearas possible, and the time it begins may be adjusted by adjusting thevalue of condenser Ei and its associated resistors. Pulses I and 2occurring before time T2 are insufficient to cause tube 25 to conductand return the multivibrator to its original state. However, any timeafter time T2, the pulse 3, no matter what its time displacement betweenmodulation limits 3a and 3b, =Will be suflicient to cause tube 25 toconduct since pulse 3 will coincide with the rising curve 40 of waveform38 and therefore is sufficient to raise the potential of the grid oftube 25 above the lcritical level 4| at which tube 25 begins to conduct.

From the foregoing it will be seen that only the marker pulses 9 andsignal pulses 3 will affect the multivibrator 2 I. While pulses 4-8occurring after the multivibrator has returned to its origina1 level ofstability, are not likely to produce any eiect in the output, to makedoubly certain that tube 25 does not act as an amplier and feed pulses 48 through to the output, the grid oi tube 25 is made so positive byapplying energy from the positive side of a source oi potential 42thereto, that normally when multivibrator is not vibrating tube 25 issaturated. The waveform 38 of curve B has been illustrated as having ahigher potential than the rectangular waveform 31 (curve B, Fig. 3) toindicate to the potential from source 42 applied to the grid oi tube 25.Tube 25 being saturated the pulses 4 8 applied over line I4 to the gridof tube 25 produce no effect in the output thereof.

In accordance with a further feature of the present invention, to guardagainst any of the pulses prior to the selected pulse returning themultivibrator to its original state after it has been tripped by theassociated marker pulse 9, the following additional means are provided.In the plate circuit of tube 24, there is provided an adjustable tunedtank circuit 43 which may be in the form of a permeability-tunednductance coil 44, (which for example may be tuned by moving slugs inand out of said coil to vary its permeability) said coil having aninherent capacity 45 making it a resonant or tuned circuit. Coil 44 isshunted by a rectifier 4S. When the positive marker pulses are appliedto the grid of tube 24, thev anode goes negative, thereby shock-excitingtank circuit 43 and producing a negative undulation 41 (curve C, Fig. 3)The negative undulation 41 has a duration slightly less than the minimuminterval between the selected pulse and the marker pulse and slightlymore than the maximum interval between the pulse preceding the selectedpulse and the marker pulse. For example, assuming pulse 3 is the pulseto be selected, the negative undulation 41 will have a duration fromtime T1 to slightly less than time T2 but slightly more than the widestseparation of any pulse 2 from its associated marker pulse 9. Thepositive undulation which would normally follow the negative undulation41 is damped out by rectier tube 45, which preferably has as low aninternal impedance as possible to assure rapid and complete criticaldamping of the positive undulation.

The negative undulation 41 is combined with waveform 31 to produce avoltage at the anode of tube 24 as indicated in the solid line 48 incurve D, Fig. 3. At the grid of tube 25, the combined voltages producedconsist of a combination of waveform 38 and a waveform corresponding tonegative undulation 41, the resulting waveform being designated by thenumeral 49. From an examination of waveform 49, it will be seen thatthere is very Aslight possibility of any pulse occurring before time T2lto have suilicient amplitude to cause tube 25 to conduct and return themultivibrator to its original level after it has been tripped by markerpulse 9. The undulation 41 thus serves as a blocking voltage tosafeguard against any accidental returning of the multivibrator to itsoriginal level. Between times T2 and T4, however, pulses will be capableof causing`the multivibrator to return to its original position becausethese pulses will cause the level of the grid of tube 25 to reach thecritical level 4| at which the tube conducts. Assuming that pulse 3 isin its middle or unmodulated position at "time T3, the multivibratorwill be returned to its original level at time T3 and the resultingoutput voltage will have a duration equal to time T1 to T3 and having arectangular waveform 50. If pulse 3 is in position 3a at one extreme ofmodulation at which it is nearest to the marker pulse, the resultingoutput voltage Will have a rectangular waveform as shown in curve E at5|. Finally, if pulse 3 is at its other extreme of time modulation, therectangular waveform of the resulting voltage will be that designated bynumeral 52. Accordingly, voltages of rectangular waveforms of differentduration, depending upon the time modulation of the signal pulses willbe produced. These voltages will have diiferent energy content and anyone of various suitable utilization means, which may include forexample, an integrating circuit 22, may be used to respond in accordancewith the energy content of these rectangular waveforms.

It will be apparent that by adjusting various elements in multivibrator2| such as for example, variable condenser 3|, resistor 34 and tankcircuit 33, etc., the multivibrator 2| may be made to select any desiredchannel. To enable this to be accomplished readily, the variableinductance 44, condenser 3| and resistor 34 may be ganged together toenable simultaneous tuning of these elemenits in one manual movement.Thus a single selector circuit may be employed to successively selectdifferent ones of a number of pulse signal channels. If, however, it isdesired to simultaneously receive several channels, then an equal numberof selector circuits, as illustrated in Fig. 1, may be employed, each ofsaid selector circuits being tuned to a separate channel.

While we have described the selector circuit as being primarily Iamultivibrator, it will be apparent to those versed in the art that othertypes of circuits, such as various forms of flip-flop circuits, may beused in place thereof.

While We have described above the principles of our invention inconnection with specic apparatus, it is to be clearly understood thatthis description is made only by way of example and not as a limitationon the scope of our invention as set forth in the objects hereof.

We claim:

1. In a receiving system for receiving a desired one of a plurality ofchannels leaved in sequence in the form of a single multichannel pulsetrain, the pulses of one of the channels being provided with anidentifying characteristic distinct from the pulses of the otherchannels 'for use as synchronizing pulses, and the pulses of at leastthe selected one of said channels being time modulated signal pulses; anadjustable circuit having an output variable between two levels andadapted, after being tripped from one of said levels to its other level,normally to return to said one level after an interval selected to besubstantially equal to the maximum interval between a synchronizingpulse and an associated signal pulse ofthe selected channel, saidcircuit being capable of being returned sooner by a suitably appliedpulse occurring after a predetermined minimum time has elapsed aftersaid circuit has been tripped, said minimum time being selected to besubstantially equal to the minimum interval between a synchronizingpulse and an` associated signal pulse of the selected of pulsesinterchannel, means 4fcr applying the synchronizing pulses to saidcircuit to trip it, means for applying the other pulses of said channelsto said circuit so that each tends to return it, and utilization meansresponsive to the length o-f time said circuit remains at said otherlevel.

2. A receiving system according to claim 1, wherein said circuit is amultivibrator.

3. A receiving system according to claim 1, iurther including adjustablemeans associated with said circuit and responsive to each of saidsynchrcnizing pulses to produce a blocking voltage having a durationsubstantially equal to the minimum interval between la synchronizingpulse and an associated signal pulse of the channel to be selected.

4. In a receiving system for receiving a desired one of a plurality ofchannels of pulses interleaved in sequence in the form of a singlemultichannel pulse train, the pulses of one of the channels beingprovided with an identifying characteristic distinct. from the pulses ofthe other channels `for use as synchronizing pulses, and the pulses ofat least the selected one of said channels being time modulated signalpulses; a circuit having 'an output variable between two levels, meansfor applying the synchronizing pulses to said circuit to trip it fromone of said levels to its other level, means for applying the otherpulses of said channels to said circuit so that each tends to re-trip itto said one level, adjustable means controlled by each of saidsynchronizing pulses to produce a blocking voltage of selected durationfor blocking said re-tripping of said circuit for a selected interval,and utilization means responsive to the length of time said circuitremains at said other level.

5. A receiving system according to claim wherein said circuit comprisesa multivibrator.

6. In a receiving system for receiving a desired one of a, plurality ofchannels of pulses interleaved in sequence in the form of a singlemultichannel pulse train, the pulses of one of the channels beingprovided with an identifying characteristic distinct from the pulses ofthe other channels for use as synchronizing pulses, and the pulses of atleast the selected one of said channels being time modulated signalpulses; a multivibrator adapted to have a normal period of oscillationsubstantially as great as the maximum interval between a synchronizingpulse and the associated signal pulse of the selected channel, means forapplying the synchronizing pulses to said multivibrator to trip it,means for applying the signal pulses to said multivibrator tending tore-trip it, adjustable means associated with said multivibrator andresponsive to each of said synchronizing pulses to produce a blockingvoltage having a duration substantially equal to the minimum intervalbetween said associated signal pulse of the channel to be selected andits synchronizing pulse, and utilization means responsive to the outputof said multivibrator.

DONALD D. GRIEG. ARNOLD M. LEVINE.

